This invention relates generally to low voltage protection circuits for induction motors such as air conditioner compressors and the like.
Generally speaking, such circuits have been known and used in the prior art. FIG. 1 of the attached drawings illustrates one such conventional low voltage protection circuit wherein an air conditioner compressor motor 10 is connected through a manually operable on/off switch 12, an adjustable thermostatically controlled switch 14 and a thermally activated low voltage protector switch unit 16 to a suitable source 18 of a.c. potential. The switch unit 16 includes a heat sensitive switch 20 thermally coupled to a heater element 22, the latter being connectable directly across the source 18. Now, when the air conditioner is connected to the source 18, the element 22 draws power therefrom and is selected so as to heat up sufficiently to activate the thermally sensitive switch 20 to close the same provided the source potential is at least high enough to safely operate the compressor motor 10. However, should the source potential drop below the safe operating level for the motor 10 for any reason, the current drawn by the element 22 will also drop to thus reduce its heat output sufficiently to deactivate and open the switch 20. The motor 10 is thereby protected against possible damage which might otherwise result if a low voltage condition occurs at the source 18 sufficient to stall the motor 10 and produce damaging locked rotor current.
One difficulty which has been encountered with such prior art circuits results from the fact that the heater element is directly connected across the source at all times while the air conditioner is so connected. This is the case whether or not the air conditioner is turned on and whether or not the thermostat is calling for compressor operation. The load thus presented by the heater element upon the source is constant and can produce a significant amount of power consumption over long periods of time just to keep the low voltage protection switch activated.
By means of my invention, this and other difficulties encountered in the prior art can be substantially overcome.